Unlock Large Potentials of Standard Mud Gas for Real-Time Fluid Typing

Standard mud gas data are part of the basic mud-logging service and are mainly used for safety. Although the data are available for all wells, it is not used for real-time fluid typing due to poor prediction accuracy. We recently developed a new method based on a large in-house reservoir fluid database, which significantly improved the fluid typing accuracy from standard mud gas data. The new technology unlocks the large potential of utilizing standard mud gas data for thousands of wells. Standard mud gas data have limited gas components that can be detected confidently (usually from C1 to C3). In addition, they are raw data without recycling correction and extraction efficiency correction. Following the traditional geochemical analysis methods, some key parameters (C1/C, C1/C, Bernard ratio) have a universal threshold to distinguish gas and oil. The main reasons for poor fluid typing accuracy are due to (a) lacking C1 to C3 composition correction for wells with oil-based mud (OBM) and because (b) geochemical parameters based on C1 to C3 are field dependent. Based on the reservoir fluid database analysis, we divide the reservoir fluids from different fields into two categories. For Type I fields, there are large differences between C1 through C3 component ratios for oil and gas. When water-based mud (WBM) is used, C1 to C3 component ratios from standard mud gas can be utilized directly to identify oil and gas. When OBM is used, we developed a new method to achieve corrected standard mud gas composition for fluid typing using pseudo-extraction efficiency correction (EEC) based on equation of state (EOS). For Type II fields, there are severe overlapping of C1 to C3 component ratios for oil and gas. The overlapping is the main reason for the poor fluid typing accuracy. We recommend utilizing a heated degasser when drilling into Type II fields to provide additional data on C4 and C5 for accurate fluid typing. Johan Castberg is a Type I field, and we achieved excellent fluid typing results for 14 wells using WBM and OBM. There is no additional data acquisition cost for standard mud gas data, which are available for all wells. The new method makes accurate fluid typing possible for real-time well decisions like well placement, completion, and sidetrack. The innovation created significant business opportunities based on the standard mud gas, which has been regarded as not applicable data for accurate fluid typing for many decades.